Defect inspection method and apparatus therefor

ABSTRACT

A defect inspection apparatus for inspecting a fine circuit pattern with high resolution to detect a defective portion is constructed to have an objective lens for detecting an image of a sample, a laser illumination unit for illuminating the sample through the objective lens, a unit for reducing the coherence of the laser illumination, an accumulation type detector, and a unit for processing the detected image signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/295,909filed on Nov. 18, 2002 now U.S. Pat. No. 6,819,416, which is acontinuation of application Ser. No. 09/797,597 filed on Mar. 5, 2001,now U.S. Pat. No. 6,556,290. The contents of application Ser. Nos.10/295,909 and 09/797,597 are hereby incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to pattern inspection andforeign substance inspection technology for detecting defects (such asshort circuit and disconnection) and foreign objects on a pattern beingexamined, and particularly to a defect inspection method for examiningdefects and foreign objects on a pattern such as semiconductor wafer,liquid crystal display and photomask, and to a defect inspectionapparatus using that method. In the following description, it is assumedthat the defect includes a foreign object.

There is known an inspection apparatus of this kind. As disclosed inJP-A-7-318326 (prior art 1), while a pattern to be examined is beingmoved, an imager such as line sensor is used to detect the image of thepattern, and the detected image signal and an image signal a certaintime delayed from that signal are compared in their brightness so that adisagreement therebetween can be recognized as a defect. In addition,another example is disclosed in JP-A-8-320294 (prior art 2).

According to the prior art 2, when a pattern to be inspected is asemiconductor wafer of which the chip areas each have, in a mixed state,high-density pattern regions such as memory mats, and low-densitypattern regions such as peripheral circuits, the detected analog patternimage signal is converted to a digital image signal, and furtherconverted to a signal of gradations so that the high-density andlow-density regions have a certain brightness or contrast ratio decidedfrom the brightness frequency distribution of the detected image, andthis gradation image signal is aligned with and compared with a separategradation image signal so that fine defects can be examined with highaccuracy.

In the recent LSI production, the circuit pattern formed on the waferhas been developed to a very fine pattern such as a line width of 0.25μm or below in accordance with the needs for higher integration. Thispattern width is the resolution limit of the image-forming opticalsystem. Therefore, the image-forming optical system is advancing towarduse of high NA (numerical aperture) and ultra-high optical resolutiontechnology.

However, the high NA has reached the physical critical limit. Therefore,the wavelength of light to be used for the detection should be reducedto ultraviolet light (UV light or DUV light) region as an essentialapproach.

Moreover, since the inspection is required to be fast conducted, themethod of scanning on a sample by fine laser beam cannot be used. If thelaser beam is spread out up to full field of view in order to illuminateat a time, speckles occur, and overshoot and undershoot called ringingare caused at the edges of the circuit pattern, thus degrading thepicture quality.

SUMMARY OF THE INVENTION

The present invention, in order to solve the above problems, is toprovide a method and apparatus for fast examining a fine circuit patternwith high resolution to detect defects thereon.

According to the invention, a laser source is used as a light source,and means for suppressing the laser speckle from occurring is providedin the light path so that coherency-reduced light is irradiated on theobject surface to examine the object image.

According to the present invention, as the means for suppressing thelaser speckle from occurring, means is provided to gather rays of lightfrom the light source at a point or a plurality of points on the pupilof an objective lens and to scan those points on the pupil in timingwith the accumulation time of a detector.

Moreover, in order to improve the pattern contrast, considering that thepolarized state of laser can be freely controlled, the orientation ofthe polarization of illuminating light and ellipticity are controlled sothat the polarized component as part of detected light can be detected.

In addition, a plurality of laser beams are used as light sources,because it is possible to expect not only the increase of the defectdetection sensitivity, but also other various effects such as long lifeand countermeasure against breakdown. Also, laser beams of differentwavelengths are used and combined because they are effective forcontrolling the polarized state. Since each laser output can also bereduced, the life of the laser sources can be extended. The addition oflaser beams is made by use of a polarization beam splitter, dichroicmirror or half mirror.

The beams of the same wavelength can be processed, by a polarizationbeam splitter, to be laser beams of which the polarization directionsare perpendicular to each other. The dichroic mirror is able to changethe polarization directions of laser beams of different wavelengths toparallel or orthogonal directions. As compared with the half mirror,those processes can be achieved with high efficiency. In addition, thepolarized state of one or both of different-wavelength beams can bechanged by use of a wave plate.

According to the invention, a laser source for ultraviolet (UV) laserbeam is used. Here, UV light and DUV are generally called UV light.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the construction of an apparatus forexamining a pattern being inspected to detect defects thereon, accordingto the invention.

FIG. 2 is a graph showing the emission spectrum of discharge tubeillumination.

FIG. 3 is a plan view showing the situations in which the pupil of theobjective lens and the field of view are illuminated by discharge tubeillumination.

FIG. 4A is a plan view showing the situations in which the pupil of theobjective lens and the field of view are illuminated by laserillumination.

FIG. 4B is a cross-sectional view of a pattern on the field of view.

FIG. 4C is a diagram showing a detected signal produced as a result ofdetecting the pattern (FIG. 4B) on the field of view.

FIG. 5 is a plan view showing the situations in which the pupil of theobjective lens and the field of view are illuminated by laserillumination that is spread on the pupil.

FIGS. 6A through 6C are plan views showing the situations in which thepupil of the objective lens and the filed of view are illuminated bylaser illumination according to the invention.

FIG. 7 is a plan view showing the relation between a CCD detector andthe illuminated region on the field of view according to the invention.

FIG. 8 is a plan view showing the relation between the CCD detector andthe illuminated region on the field of view according to the invention.

FIG. 9 is plan views showing the CCD detector and the situations inwhich the pupil of the objective lens and the field of view areilluminated by laser illumination according to the invention.

FIG. 10 is plan views showing the TDI detector and the situations inwhich the pupil of the objective lens and the filed of view areilluminated by laser illumination according to the invention.

FIG. 11 is a substantially front view of the laser source to whichreference is made in explaining a conceptual model for reducing thespatial coherence of laser illumination according to the invention.

FIG. 12 is a substantially front view of the laser source to whichreference is made in explaining a conceptual model for reducing thespatial coherence of laser illumination according to the invention.

FIG. 13 is a substantially front view of the optical system to whichreference is made in explaining a conceptual model for reducing thespatial coherence of laser illumination according to the invention.

FIG. 14 is a substantially front view of the laser source to whichreference is made in explaining a conceptual model for reducing thespatial coherence of laser illumination according to the invention.

FIG. 15 is a substantially front view of the laser source to whichreference is made in explaining a conceptual model for reducing thespatial coherence of laser illumination according to the invention.

FIG. 16 is a perspective view of the apparatus for examining the patternto detect defects thereon according to the invention, by synchronizingthe stage, pupil scanning optical system and sensor.

FIG. 17 is a front view of an optical system to which reference is madein explaining the situation in which light of laser illuminationaccording to the invention is converged on the pupil of the objectivelens.

FIG. 18 is a graph showing the intensity distribution of beam from thelaser source.

FIG. 19 is a front view of an embodiment of the mechanism for scanningthe pupil by the laser illumination according to the invention.

FIG. 20 is a front view of an embodiment of the mechanism fortwo-dimensionally scanning the pupil by the laser illumination accordingto the invention.

FIG. 21 is a plan view of an embodiment of the mechanism for scanningthe pupil by the laser illumination according to the invention, with adiffuser inserted in the light path.

FIG. 22 is a perspective view of a group of glass rod lenses accordingto the invention.

FIG. 23 is a perspective view of a multi-cylindrical-lens arrayaccording to the invention.

FIG. 24 is a substantially front view of an embodiment of the opticalsystem according to the invention.

FIG. 25 is a front view of an embodiment of the optical system having amechanism for controlling the polarized state of the laser illuminationaccording to the invention.

FIG. 26 is plan views to which reference is made in explaining thesituations in which the pupil is illuminated by a bracelet-likeillumination according to the invention.

FIG. 27 is front views of the TDI image sensor according to theinvention.

FIG. 28 is a block diagram of the construction for comparing imagesaccording to the invention.

FIG. 29 is a substantially front view showing the construction fordetermining defects according to the invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the defect inspection method and apparatus according tothe invention will be described with reference to the accompanyingdrawings. FIG. 1 is a diagram showing one embodiment of the apparatusaccording to the invention. Referring to FIG. 1, there is shown an X, Y,Z, θ (rotation) stage 2 on which a semiconductor wafer 1 is placed asone example of the pattern being examined. There are also shown anoptical system 7 including the objective lens, and illumination lightsource 3 (for example, UV laser sources of 266-nm wavelength and 257-nmwavelength) for illuminating the semiconductor waver 1. Here, while twolaser sources are shown, a plurality of laser sources may be provided.In addition, the two wavelengths may be different or equal. These lasersources, when combined, generate various different effects. However, theoptical system is corrected for wavelength according to the wavelength.

There is also shown a beam splitter (in some case, it may be apolarization beam splitter or half mirror) 5 that reflects theillumination light from the light source 3 into the optical system 7 sothat, for example, bright field illumination is performed on thesemiconductor wafer 1 through the optical system. Shown at 6 is a waveplate that is formed of a ½ wave plate and ¼ wave plate. There are showna coherent reducing mechanism 4 which is, for example, a scanningmechanism for scanning the pupil of the objective lens 7 by the laserbeam from the laser source, and an image sensor 8 which generates ashading image signal according to the brightness (shading) of thereflected light from the semiconductor waver 1 as an example of thepattern.

While the semiconductor wafer 1 on the stage 2 is being rotated at anconstant speed, and scanned, the image sensor 8 detects the brightnessinformation (shading image signal) of the pattern formed on thesemiconductor waver 1.

Shown at 12 is an image processing system which compares the dies andthe repeated patterns within a die. Here, the detected image is comparedwith an image delayed by an amount corresponding to the cell pitchproduced from a delay memory 11. The coordinates of the array data onthe semiconductor wafer 1 are previously entered through input device(not shown) of keyboard or disk, and a control system 13 generatesdefect-examined data and forces it to be stored in a memory (not shown)according to these coordinates of array data. The defect-examined data,if necessary, can be displayed on displaying device such as a nonitordisplay and fed to output device.

The detailed construction of the comparator may be that disclosed inJP-A-61-212708. For example, it is formed of an alignment circuit forimages, a differential image detector circuit for aligned images, adisagreement detection circuit for converting a differential image to abinary value and a feature extraction circuit for calculating the area,length (projected length) and coordinates from the binary output.

The light source 3 will be described. For high resolution, it isnecessary to select short wavelengths. In the UV wavelength region thatprovides the best effect for high resolution, it is difficult to obtainhigh illumination intensity. Discharge lamps are excellent as UV lightsource, and particularly mercury xenon lamp has brighter line spectra inUV region than other discharge tubes.

FIG. 2 shows one example of the radiation intensity of mercury xenonlamp relative to wavelength. The bright line spectra in DUV region areonly 1˜2% of all output light (the visible light region is about 30%) ascompared with the wide visible wavelength range that is used in theprior art). Also, the light radiation is not directional. The efficiencyat which the light emitted from the discharge lamp can be led up to thesample cannot be increased even in a carefully designed optical system.Thus, the illumination by the discharge lamp for the UV region cannotoffer a sufficient amount of light in the use for fast image detection.

Even if a high-power discharge lamp is used for high illumination(brightness) on the sample, only the luminescent spot size is largerthan when a small-power one is used, and thus the brightness (lightpower per unit area) cannot be increased. Therefore, the laser source issuited for high brightness UV illumination.

Thus, the laser for light source has a large merit. The presentinvention employs this laser illumination.

FIG. 3 shows the situations in which the objective lens pupil and thefield of view are illuminated by normal white light. In FIG. 3, ASindicates the pupil, and FS the field of view. The image of the lightsource is formed at 31 as a formed image, and the field of view issubstantially uniformly illuminated as at 32.

FIG. 4A shows the illumination by a laser source. In this case, thelaser source image is formed at a point 41 on the pupil as shown in FIG.4A. The circuit pattern illuminated as a scan 42 over the field of view,if it is such a cross-sectional pattern as shown in FIG. 4B, is detectedto be a waveform as shown in FIG. 4C. When the circuit pattern isilluminated by a laser source and detected to produce the image of thecircuit pattern as described above, overshoot and undershoot are causedat the edges of the pattern, and speckle occurs because the σ of theillumination is small. It can be considered that the illumination overthe field of view below the objective lens is not performed from variousangles relative to the sample. Under the usual white light source, thepupil of the lens is illuminated over a sizable area, and the field ofview toward the sample below the lens is illuminated over from a wideangle range equal to the NA (numerical aperture) of the objective lens.

The σ of the coherent light such as laser light is 0 (since σ isproportional to the area of light source image on the pupil). Since thecoherent light is from a point light source, the image formed on thepupil is a point. Of course, laser light can be spread by anotherseparate lens system to form a light flux 51 on the pupil as shown inFIG. 5. However, since the laser source has coherence, the same thing aswhen all light rays are irradiated from the position of σ=0 occurs as aresult 52 shown in FIG. 5, and thus the problem cannot be solved yet.Therefore, it is necessary to use the means for reducing the coherenceof laser. In order to reduce the coherence, it is necessary to selecteither time coherence or spatial coherence and reduce.

Thus, according to the invention, the light source image is formed onthe pupil of the objective lens of the inspection apparatus, and thisimage is moved to scan the pupil, for example, first at position 61,next at the position 62, then position 63, . . . so that the field ofview can be illuminated over like lines 65 as shown in FIG. 6A. Theimages of speckle, overshoot and undershoot occur at each position, buthave no coherence since they are caused at different times. Thus, whenall images as a result of scanning are added on the detector, the sameimage as the coherent light source can be obtained. A detector of theaccumulation type, such as CCD is suited for the addition on thedetector.

FIG. 19 shows an example of the construction for scanning the laser spoton the pupil of the objective (detection/illumination) lens according tothe invention. In this figure, only the illuminating side structure isshown, the detecting side construction being not shown. For the sake ofexplaining the principle, only the one-dimensional scanning mechanism isshown.

The beam (which is parallel because of laser beam) from a laser source197 is shaped into a necessary beam shape by a beam shaping mechanism196, and deflected by a scanning mechanism 195. Here, a polygon mirroris shown as an example of the scanning mechanism. The deflection angleof the deflected parallel beam is converted to the change of position byan f-θ lens 194 called condenser lens. Thus, the lens 194 is placed at aposition separated by the focal distance of the lens 194 away from thescanning mirror surface. The beam from the lens 194 is converged on apupil plane 193 of an object lens 192. Therefore, the distance betweenthe lens 194 and the pupil plane 193 is also equal to the focal distanceof the lens 194. Thus, the laser beam from the object lens 192 is, whileits direction angle is being changed, irradiated on the sample, 191.

FIG. 20 shows the case in which the laser beam is deflected to scan thepupil in a two-dimensional manner. In this figure, a plate-like mirrorsuch as a galvano mirror is shown as one example of the scanningmechanism. It may be a movable mirror produced by a micro machine. Inaddition, a mirror 1911 shown in the figure is used to bend the lightpath, and thus is not essential. Therefore, this structure is differentfrom the construction shown in FIG. 19 in that an f-θ lens 199, ascanning mirror 198 as the scanning mechanism for one more axis, and anincidence lens 1910 to the scanning mirror 195 are added. Theconstruction shown in FIG. 20 can achieve the two-dimensional scanningshown in FIG. 6.

The NA of the objective lens 192 in this embodiment is 0.75. The largerthe NA, the greater the pupil scanning effect, thus reducing theinfluence of light interference caused by the thin film patterns (sincethe brightness of pattern depends on its film thickness, so that thedifference of a defective portion to the correct portion becomes greatin the pattern comparison which will be described later, it is difficultto detect a fine defect. Even if there is a very small thickness changecalled “grain” or “hillock”, the brightness change becomes great).

FIG. 21 shows an example of the construction having a diffuser placed inthe light path. The diffuser is placed at a conjugate position to thepupil 193 of the object lens 192. In this example, since the laser beamis deflected to scan the diffuser, the coherence reducing effect isgreater. Of course, the diffuser may be fast reciprocated or rotated inthe direction perpendicular to the optical axis of the laser beam.

The light source image is formed on the pupil by converging the laserbeam from the laser source through a condenser on a pupil plane 171 ofan objective lens 172 as shown in FIG. 17 (when image detection is madeunder the bright field illumination, the illumination or irradiationlens and the detection lens are shared by one lens). Here, the laserbeam from the laser source as a point source forms a spot when convergedup to the diffraction limit. In other words, all the laser beam outputis concentrated to this spot, and thus the power at this spot isconsiderably large.

The actual objective lens is formed of a large number of lenses (at mostten lenses) in order to compensate for the aberration. Thus, the pupilplane 171 is located at a position not only away from the lens, but alsowithin the lens (glass material portion) or near the lens surface,depending upon the design of the objective lens. In this case, thecoating (for reflection prevention) formed on the lens may be damagedwhen it is exposed to high-power laser beam, thus causing a seriousproblem. The confocal laser scanning microscope usually called laserscanning microscope makes the laser beam spread on the pupil plane,while the present invention forms the spot on the pupil plane (on theother hand, the laser scanning microscope narrows the spot down on thesample, and thus there is the possibility that the sample is damaged).

In the present invention in which the pupil plays a great role for theobjective lens, the pupil plane position is previously set to beseparated from the lens surface, thereby avoiding the problem fromoccurring. When the pupil plane is separated from the lens surface, thespot is out of focus, and hence its diameter is slightly larger with theresult that the average power density is lowered.

Moreover, when the pupil plane cannot be separated enough away from thelens surface because of the object lens structure, only that lens can beallowed not to be coated. The inventors think that if only a componentof the lens is not coated, the effect of all the object lens on thetransmissivity is small, and that the proof strength of the coatings inthat case can be kept enough.

In addition, a continuous oscillation type laser should be desirablyselected as the laser source. The reason for this is that the pulseoscillation type laser generates a very high power laser pulse output(peak), thus leading to a damage to the lens even though the averageoutput can be restricted to a low value. Of course, a small-output laserfree from care about damage may be of the pulse oscillation type.

The laser spot thus formed on the pupil may be moved to scan by spiralscanning 66 or television (raster) scanning 67 as shown in FIGS. 6B and6C, or by other scanning operations. In this case, however, it isdesired to make the unit scanning within the accumulation time of thedetector. Therefore, the scanning should be synchronized with theoperation of the detector. For example, referring to FIG. 20, when thepupil plane is scanned in a ring shape (such scanning as shown in FIG.26, which will be described later), the galvano mirrors 195, 198 may bedriven with a fundamental period of 1 kHz if the accumulation time ofthe image sensor is 1 m sec. In addition, the stage, the sensor and thepupil plane scanning should be synchronized with each other. In thiscase, the stage has the largest inertia, and thus it is most difficultto synchronize.

The pupil plane scanning optical system can easily synchronize over awide frequency range or a limited frequency range depending upon thetype of the mechanism. Moreover, since the sensor is formed of anelectric circuit, the synchronization is easy. Thus, if the basicsynchronizing signal is generated from the position of the stage, thetwo other portions can be easily synchronized with the stage. Thismethod is desirable.

FIG. 16 shows that system. The stage position is determined by aposition detection mechanism 161 such as a linear encoder mounted on theXY stage 2, and fed to a synchronizing signal generator 163, which thengenerates a sync signal 164 such as a sensor transfer pulse, and a syncsignal 165 for the pupil plane scanning mechanism.

The pupil plane scanning mechanism can be most easily synchronized whenan electric signal to the A/O deflector or E/O deflector is converteddirectly to the deflection angle of light. The deflector may be the typein which a galvano mirror or polygon mirror is used as a basis.

Thus, the image of illumination 65 can be obtained over the field ofview as illustrated in FIG. 6A at FS. Here, use of two wavelengths hasthe effect of reducing the coherence.

The laser beam synthesizer 10 will be described. Two laser beams aresynthesized by use of, for example, a polarization beam splitter (PBS).In this case, if the polarization directions are made perpendicular, thesynthesis can be made with high efficiency. The two laser wavelengthsmay be different or equal. A dichroic mirror may be used instead of thepolarization beam splitter. In this case, the two laser wavelengths areassumed to be different. The dichroic mirror synthesizes the beams byuse of the difference between the wavelengths. In this case, thepolarization directions may be equal or parallel. The half mirror mayuse the beams of different polarization directions, but reduces theefficiency of the synthesis. Also, it can be considered that the ½ waveplate 6 corresponding to the wavelength is placed, and the polarizationdirections are made equal by use of the difference between thewavelengths. In any case, it is possible to achieve the most suitableconstruction considering the efficiency and polarization of theillumination.

Let it be considered that a one-dimensional sensor is used as theaccumulation type detector. As illustrated in FIG. 7, even if all thefield of view is illuminated over against a one-dimensional sensor 71,only a region 72 can contribute to the detection. A region 73 thatoccupies the most part of the light power does not contribute to thedetection. In order to increase the illumination intensity, it isdesirable to use linear illumination as indicated by a region 82 againstthe one-dimensional sensor 71 as shown in FIG. 8. (The CCD scans in theY-direction on the field of view to thereby produce a two-dimensionalimage.) In that case, when the pupil plane is illuminated in thelongitudinal Y-direction as shown in FIG. 9 at 91, the field of view isilluminated over so that the illumination conforms to the shape of theCCD 71 as shown at 92. In addition, the pupil plane is scanned in theX-direction. The period, Ts of the scanning is shorter than theaccumulation time, Ti of the CCD. Thus, the images can be added.

The problem is that since the illumination on the pupil is spread out inthe Y-direction from the start, the pupil cannot be scanned in theY-direction. Therefore, it is impossible to reduce the overshoot andundershoot that the CCD causes in the Y-direction on the field of view.On the contrary, if the length of the illumination in the Y-direction onthe pupil is tried to reduce in order for the pupil to be scanned in theY-direction, the width of the illumination in the Y-direction on thefield of view will be spread out, thus the illumination intensity beinglowered.

According to the invention, this problem can be solved by using a TDI(Time Delay & Integration) type sensor of CCD sensors as shown in FIG.10. Since the TDI sensor placed on the field of view has N-stage (N is anumber of stages such as dozens of stages ˜about 256 stages.)light-sensitive portions arranged thereon, the illumination light can bedetected effectively even if the width of the area illuminated in thefield of view is spread N times the normal width.

Therefore, the length of the illumination spot, 102 on the pupil in theY-direction can be reduced to about 1/N that of CCD, so that the pupilcan be scanned in both X-direction and Y-direction. Thus, the overshootand undershoot that the TDI causes in both X-direction and Y-directionon the field of view can be decreased, and hence the images can besatisfactorily detected. In addition, the period Ts of scanning thepupil is required to be shorter than N times the accumulation time ofone stage of the TDI. However, for more uniform detection consideringthe illumination intensity distribution on the field of view, thescanning period Ts should be selected to be shorter than N/2 times theaccumulation time Ti of CCD. In addition, for uniform illumination, thelight from the laser source is not converged directly on the pupil, butdesirably through a fly eye lens or integrator thereon.

A method of reducing the spatial coherence will be described. In orderto reduce the spatial coherence, it is necessary to acquire light raysthat have a longer light-path difference than the coherence length oflaser. More specifically, if the laser beam is incident to a bundle ofoptical fibers, 111 or glass rods of which the lengths are different asshown in FIG. 11, the output light becomes incoherent (coherent-free).If these fibers are placed on the pupil, the images obtained have noovershoot and undershoot. In this method, the coherence length of lasersource should be shorter, and for this purpose the laser source shouldbe selected to oscillate a plurality of longitudinal modes of a widerband Δλ₂ shown in FIG. 11 at b) rather than a single longitudinal mode(oscillation spectrum) of narrow band Δλ₁ in FIG. 11 at a).

Another idea for reducing the spatial coherence employs the phenomenonthat when light rays with the optical axis shifted are incident to theoptical fibers, the lateral mode (spatial distribution, light intensityI) of the exiting light rays changes. This mode change is usuallyconsidered to be unfavorable for industrial applications, and it is ageneral practice to make efforts toward the reduction of the lateralmode change. The present invention counters this mode change byintentionally shifting the optical axis in various ways and making thelight rays incident to fibers 121 so as to produce exiting light raysa), b), c), d), e) . . . with the lateral mode changed differently asshown in FIG. 12. Consequently, the produced exiting light rays areincoherent, and thus irradiated on the pupil. In this method, a verylarge number of light sources (spots on the pupil) can be produced bybundling a plurality of fibers.

FIG. 13 shows the situation in which the beam emitted from the lasersource 3 is split by a polarization beam splitter 131 into two laserbeams 133, 134 that have polarized planes perpendicular to each other.Shown at 132 is a mirror for changing the direction of the beam. Sincethe beams having the perpendicular polarized planes have no coherence,light rays with no coherence can be obtained by a very simpleconstruction. In this method, only two light rays can be produced, butrays with no coherence can be-obtained with ½ labor hour by combiningwith the previously mentioned method.

In addition, since every light sources independent of each other have nocoherence, independent light sources 141, 142, 143, 144 . . . may beused to illuminate the pupil of the objective lens 7 at differentpositions as shown in FIG. 14. Moreover, by combining with the ideausing the polarization beam splitter, it is possible to reduce thenumber of laser sources to ½ the original number, and thus the cost isalso reduced.

Although we have described so far a plurality of ideas, or methods ofreducing the laser coherence, illuminating the pupil at a plurality ofpoints and converting the illumination light rays through the objectivelens to form the image, these methods can be combined or the equivalentsto those methods may be used to reduce the coherence.

Moreover, when a vibrating (or trembling) mirror or the like is placedin part of the light path so that part of the illumination light ischanged in its path for laser illumination and when the images formed bylight rays passed through different light paths are accumulated on atime basis so that the image can be detected, the time coherencereduction action can be made in that process, and hence it is notnecessary to reduce the spatial coherence so strictly as mentionedabove.

Specifically, when a plurality of spots are formed on the pupil plane,it is not necessary to provide a light path difference beyond thecoherence length. As, for example, shown in FIG. 22, a group of glassrod lens (fly eye lens) having uniform length may be used to generate aplurality of light sources from a single laser source. Moreover, amulti-cylindrical lens array that is simpler in its structure than theglass rod lens group may be used as shown in FIG. 23. Since themulti-cylindrical lens array generates a plurality of light sources onlyin one direction, a plurality of light sources can be two-dimensionallygenerated by use of two those arrays placed perpendicularly to eachother. In that case, by changing the cylinder pitch of each array, it ispossible to generate a group of light sources of which the lateral- andlongitudinal-array ones respectively have different pitches.

The additional advantage of this method is that, as for example shown inFIG. 26A, when a light source group 252 with its magnification changedis formed on a pupil plane 251 and rotated to scan in a ring-like shapeas for example indicated by arrow 254 in FIG. 26A, an on-pupilillumination distribution 253 can be obtained as shaded in FIG. 26B,leading to the ring-band illumination by which the detected imageresolution can be improved. In addition, only by changing the magnifyingpower of the light source group, it is possible to change the ring-bandillumination condition. All the pupil plane can be illuminated in orderto satisfy σ=1.

Use of the N-stage TDI image sensor of which the scan rate is 1 kHzfurther adds an advantage. The fundamental period of the galvano mirrormay be 1 kHz/N, with which all the pupil plane can be scanned. Thegalvano mirror of a few kHz is available, and the combination of thismirror and the TDI image sensor will make it possible to scan the pupilat a practical speed, leading to fast image detection. Here, the numberof stages in the TDI image sensor (the number of stages) is selectedaccording to the speed of the galvano mirror. Also, by using astage-variable TDI image sensor, it is possible to change theaccumulation time according to the pupil scanning method.

FIG. 24 is a diagram showing an illumination system using the above lensarray. Although the construction should be shown in a three-dimensionalmanner as shown in FIG. 21, it is schematically shown here for thepurpose of explaining about the important convergence of light. Aparallel light flux 235 from a laser source is made incident to a lensarray 234 so that a plurality of spots (new light sources) are formed ona second conjugate plane 233 that is conjugated with the pupil plane 193of the objective lens 192. Then, a plurality of light flux exit, but forthe sake of better understanding, a single light flux is shown in FIG.24. The light rays exiting from the light source group are converted tosubstantially parallel light flux by a second projection lens 232, andprojected on the second scanning mirror surface 198.

The light reflected from the second scanning mirror surface 198 ispassed through a first pupil conjugate plane 231 by the second condenser199, and it is converted to substantially parallel light and projectedon a first scanning mirror surface 195 by the first projection lens1910. Then, the light is converged on the pupil plane 193 by the firstcondenser 194, and converted to substantially parallel light andirradiated on the sample surface 191 by the objective lens 192. Theadvantage of this method is that since the plurality of formed spotshave outputs corresponding to the intensity distribution of the incidentGaussian beam 235, those spots are superimposed on each other on thesample surface 191 to act as illumination with less illuminationintensity distribution.

A description will be made of a method for improving the contrast of thepattern in addition to the increase of the resolution by use of shortwavelength.

We considered that the polarized state of laser can be freelycontrolled, and made it possible to detect the polarized component aspart of the detected light by controlling the polarization direction andellipticity of the illumination light in order to improve the patterncontrast. FIG. 25 shows an improved one of the illumination opticalsystem of FIG. 1.

One of the features of laser illumination is linear polarization. When adichroic mirror is used in the laser beam synthesizer 10, the laser beamcan be polarized in the same direction. Thus, the polarized state can beefficiently controlled by a polarizer 241 such as ½ wave plate and ¼wave plate provided in the light path. For example, the polarized statecan be controlled by rotating the ½ wave plate and ¼ wave plate aroundthe optical axis.

Since the pattern contrast is greatly changed by the change of thepolarized state of illumination, the performance of the optical systemcan be improved by making the polarized state controllable (positioningthe wave plate by rotation). More specifically, the linear polarizationdirection can be controlled by the ½ wave plate, and the ellipticity canbe changed by the ¼ wave plate. In addition, a desired polarizedcomponent can be extracted by an analyzer 242 provided on the detectionside. The component that does not contribute to defect detection, forexample, 0-order light, can be more reduced, and the component thatincludes pattern edges such as diffracted light and that contributes todefect detection can be much extracted. Thus, the detection sensitivitycan be increased.

The analyzer should be made rotatable in association with the polarizedstate. By combining these methods, it is also possible to achieveparallel nicols and crossed nicols. Of course, circularly polarizedstate can be obtained. These states do not depend upon the illuminationwavelength itself. If the above concept comes into existence, the actualconstruction may be arbitrary.

When a polarized beam splitter is used for the laser beam synthesizer10, the laser beam is, for example, polarized substantially in theperpendicular direction. Thus, the perpendicularly linearly polarizedbeams are controlled in their polarized states by the polarizer 241 of ½wave plate and ¼ wave plate.

When the diffracted light from the pattern is observed on the pupilplane of the objective lens (though not shown in FIG. 25, apupil-observing system is easily provided), more reduction of 0-orderlight than high-order diffracted light can be confirmed by selecting apolarized state. Thus, it is possible to attenuate the low-frequencycomponent and thus improve the pattern contrast. Of course, a spatialfilter may be provided at a position conjugated with the pupil of theobjective lens so that the 0-order light can be reduced (the spatialfilter can block the diffracted light from the pattern and lead thescattered light from a foreign object to the image sensor). However, bycontrolling the polarization, it is possible to efficiently extract thehigh-order diffracted light. The experiment by the inventors shows thatthe contrast can be improved about 20˜300%.

Moreover, the polarizer 241 can be provided at a position where adesired performance can be obtained (for example, between half prism 241and ¼ wave plate 6) irrespective of the position shown in FIG. 25.

FIG. 27 shows the structure of the image sensor 8 used in FIG. 1. When aDUV laser source is used, it is necessary to use an image sensor thathas enough sensitivity to the DUV. The surface irradiation type imagesensor cannot effectively detect the DUV light because the incidentlight is passed through a gate and fed to CCD with the result that theshort wavelength incident light is so attenuated that the sensitivity tothe wavelength of less than 400 nm is very low. In order to increase thesensitivity of the surface irradiation type image sensor to DUV, it isnecessary to use a method of decreasing the thickness of the gate sothat the short wavelength light can be prevented from being attenuated.

Alternatively, if cover glass is coated with an organic thin film sothat visible light can be emitted therefrom according to the incidentDUV light, the image sensor having the sensitivity only to visible lightcan be used to detect DUV light.

Moreover, since the tear-side irradiation type image sensor isconstructed so that the rear side free from gate structure can receiveincident light, it has a high quantum efficiency (for example, 30% orabove), a wide dynamic range (for example, 3000 or above), and a highsensitivity to wavelengths of 400 nm or below. Thus, it can beprofitably used particularly for illumination by wavelengths shorterthan 200 nm. This image sensor may be single even when some wavelengthsare used.

In addition, use of TDI (Time Delay Integration) type image sensor canraise the sensitivity to light. If the sensor is designed to haveantiblooming characteristic, it can solve the problem that when thedetected amount of light is more than necessary, the charges areoverflowed into the surrounding pixels. Moreover, use of MOS type imagesensor and a built-in log amplifier is effective for high dynamic range,and multi-tap construction is also effective for on-chip switching ofthe stages.

FIG. 28 is a block diagram of an example of the processing system fordetecting defects from the image fed from the image sensor. Since theobject to be inspected has repetitive patterns, prospective points fordefects are extracted by comparing the pattern being inspected, with theneighboring pattern. The output signal from the image sensor, 271 isconverted from analog to digital signal. A delay memory 272 delays thesignal from the A/D converter by the amount corresponding to one pitchin order to produce the reference image for use in the comparison. Thus,the output from the delay memory is the image corresponding to one-pitchdelay of the examined image. A comparator 273 compares those two imagesto produce the difference between the associated pixel values. Theresulting image difference is converted to a binary value on the basisof a threshold for defect detection, thus the defect prospective pointbeing extracted.

The threshold for binary conversion is previously determined ordetermined from the brightness or the like of the image being examined,and all the image is converted to a binary value by use of thisthreshold.

It can be considered to calculate the threshold at each coordinates ofthe image or at each brightness, and convert each point of the image toa binary value by use of a different threshold.

Although the image after the binary conversion includes falseinformation, features are extracted from the detected prospective pointsin order that only defects can be extracted as exactly as possible. Afeature value extractor 274 calculates the area, coordinates andprojection length of the defect prospective points. Then, decision ismade of whether the defect prospective points are actually defects orfalse information from the calculated feature values, thus finallydefect 275 being detected.

Another embodiment including an image processing operation for twoimages to be compared will be described below. In this embodiment,particularly, the brightness correction is positively performed in orderfor the two images having different brightness to be compared.

Referring to FIG. 29, there is shown the image sensor 8 (sensitive toDUV) that generates a shading image signal according to the brightnessof the reflected light, or thickness thereof from the semiconductorwafer 1 as the pattern to be examined. Shown at 9 the A/D converter bywhich the shading image signal produced from the image sensor 8 isconverted to a digital image signal 285. The digital image signal isdelayed by the delay memory 11. There are shown the stage 2 on which thesemiconductor wafer 1 as the pattern being examined is placed and movedtogether in the X-, Y-, Z- and θ-direction (rotation angle), theobjective lens 6 facing the semiconductor waver 1, and the half mirror 5that reflects the illumination light into the objective lens 6 and thenthe semiconductor wafer 1, and allows the reflected light from thesemiconductor wafer 1 to transmit therethrough. Thus, the illuminationlight from the laser source is reflected from the half mirror 5 into theobject lens 6 and then the semiconductor wafer 1 so that, for example,bright-field illumination is applied on the wafer 1. The pupil of theobjective lens 6 is scanned according to the method mentionedpreviously.

The delay memory 11 may be a memory for storing each cell or eachplurality of cell pitches of the image signal 285, thereby delaying, ormay be a memory for storing each chip or each plurality chip pitches ofthe image signal 285.

The digital image signal 285 and delayed digital image signal 284 arealigned with each other by an element block 286. Here, a positionaldeviation at which the brightness difference between associated pixelsbecomes the minimum is detected according to normalization correlation,and one of the images is shifted on the basis of this positionaldeviation so that the two images can be aligned. The normalization ismade to reduce the effect of the brightness difference between theimages.

That is, a stored image g (x, y) is moved relative to a detected image f(x, y), and a position (Δx, Δy) at which the correlation value R (Δx,Δy) becomes the maximum is calculated from the following equations (Δx,Δy: integer).

$\begin{matrix}{{R( {{\Delta\; x},{\Delta\; y}} )} = {\sum\limits_{x = 0}^{X - 1}\;{\sum\limits_{y = 0}^{Y - 1}\;\frac{\{ {{f( {x,y} )} - \overset{\_}{f}} \}\{ {{g\{ {{x + {\Delta\; x}},{y + {\Delta\; y}}} )} - {\overset{\_}{g}( {{\Delta\; x},{\Delta\; y}} )}} \}}{\sqrt{f_{\sigma} \cdot {g_{\sigma}( {{\Delta\; x},{\Delta y}} )}}}}}} & (1) \\{\overset{\_}{f} = {\frac{1}{XY}{\sum\limits_{x = 0}^{X - 1}\;{\sum\limits_{y = 0}^{Y - 1}{f( {x,y} )}}}}} & (2) \\{{\overset{\_}{g}( {{\Delta\; x},{\Delta\; y}} )} = {\frac{1}{XY}{\sum\limits_{x = 0}^{X - 1}\;{\sum\limits_{y = 0}^{Y - 1}{g( {{x + {\Delta\; x}},{y + {\Delta\; y}}} )}}}}} & (3) \\{f_{\sigma} = {\sum\limits_{x = 9}^{X - 1}\;{\sum\limits_{y = 0}^{Y - 1}\{ {{f( {x,y} )} - \overset{\_}{f}} \}^{2}}}} & (4) \\{{g_{\sigma}( {{\Delta\; x},{\Delta\; y}} )} = {\sum\limits_{x = 0}^{X - 1}\;{\sum\limits_{y = 0}^{Y - 1}\{ {{g( {{x + {\Delta\; x}},{y + {\Delta\; y}}} )} - {\overset{\_}{g}( {{\Delta\; x},{\Delta\; y}} )}} \}^{2}}}} & (5)\end{matrix}$

Here, the image is continuously detected by the image sensor, and theimage is divided into small regions each of which undergoes alignment.In the above equations, the detected image has a size of X×Y pixels. Thedivision into small regions is made in order to cope with the distortionthat the image has. In other words, the small regions are determined intheir size so that the image distortion within each small region can bealmost neglected.

Although not shown, the above-mentioned normalization correlation fordetermining the positional deviation of the image is not necessary tomake for all image, but for example, may be made for aninformation-containing one of small image sections (the size of which isX/K×Y pixels) into which the image is divided in the longitudinaldirection of the image sensor. The decision of whether informationcontains in a small image section or not is made by, for example,differentiating each small image section, detecting the presence orabsence of an edge and selecting a small image section having moreedges. If the image sensor is a linear image sensor of the multi-tapstructure capable of parallel outputs, each tap output image correspondsto the small image section. This idea is based on the fact that theparallel output images have an equal positional deviation. In addition,the normalization correlation is separately calculated for each smallimage section, the maximum positional deviation of the calculated onesfor small regions may be employed. The image sensor used here may be thetime delay integration TDI CCD of parallel output type that is sensitiveto DUV.

The image signals of different brightness are converted in theirgradations so that both the brightness values can be equal by use of agradation converter 287. Here, each pixel is linearly converted byadjusting gain and offset so that both the brightness values can beequal.

The produced image signals are compared with each other by a comparator288. If there is a disagreement as a result of comparison, it isdetected as a defect.

Each pixel of the detected image signal is converted in its gradation onthe basis of the above method, and then sequentially undergoes pipe linetype image processing. Finally, the defect and its feature are produced.

The operation of the inspection apparatus of the above structure will bedescribed.

Referring to FIG. 29, the illumination light focused by the objectivelens 6 scans the semiconductor wafer 1 as the pattern to be examinedwhile the stage 2 is being moved in the X-direction at an equal speedtogether with the wafer. The image sensor 8 detects the brightnessinformation (brightness image signal) of the pattern formed on the wafer1, or of the memory mat within a chip and peripheral circuits.

After the movement for one line, the stage is fast moved to the nextline in the Y-direction, and positioned. That is, constant speedmovement and fast movement are repeated for inspection scanning. Ofcourse, step & repeat type inspection is permissible. The A/D converter9 converts the output (brightness image signal) of the image sensor 8 tothe digital image signal 285. This digital image signal is of 10 bits.Of course, about 6 bits will cause particularly no problem with theimage processing. However, in order to detect a very small defect, it isnecessary to provide a certain number of bits.

Here, the detection and processing of the image are performed for eachpixel at 50 MHz or below. Thus, when wafer disks of 200 mm in diameterare processed, defects including a size of 50 nm or below can bedetected at a speed corresponding to a throughput of three disks perhour. Therefore, useful test information in the semiconductormanufacturing line can be produced in a reasonable time.

Thus, according to the invention, it is possible to obtainhigh-brightness illumination, image high-resolution patterns in a shorttime, and as a result, produce a high-speed, high-sensitivity inspectionapparatus. The information of the detected pattern defects are producedas their positions and sizes.

Moreover, according to the invention, a laser source advantageous forits practical application is used to provide short wavelengthsillumination essential for high resolution. The images having a highquality equivalent to or higher than the general discharge tubeillumination can be produced with high sensitivity and high speed. Thus,defects can be detected with high sensitivity.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive. The scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1. A defect inspection apparatus, comprising: a light source to emitlight having plural wavelengths; an optical unit to irradiate a sampleon which patterns are formed with the ultraviolet light having pluralwavelengths through an objective lens; a table unit to mount said sampleand movable at least in one direction; an image detecting unit to detectan image of said patterns on said sample through said objective lenswith an image sensor by adjusting a polarization condition of said imageby a polarization adjusting section to improve contrast of said image ofsaid patterns and parallelly outputting a signal detected by said imagesensor, wherein the image sensor is a time delay and integration (TDJ)sensor, and wherein a period Ts of scanning a pupil is required to beshorter than N times an accumulation time of one stage of the TDIsensor, where N is an integer; and an image processor to process adetected image by using a reference image and to detect a defect on saidsample.
 2. A defect inspection apparatus according to claim 1, whereinsaid image sensor is a parallel-output type image sensor.
 3. A defectinspection apparatus according to claim 1, wherein said image sensor iscapable of producing 64 channels or more parallel outputs.
 4. A defectinspection apparatus according to claim 1, wherein said image sensor isa rear-side irradiation type in which the rear side is sensitive toultraviolet light.
 5. A defect inspection apparatus according to claim1, wherein said time delay integration sensor is a rear-side irradiationtype in which the rear side is sensitive to ultraviolet light.
 6. Adefect inspection apparatus according to claim 1, wherein said imagesensor is sensitive to a wavelength of less than 400 nm.
 7. A defectinspection method, comprising: emitting ultraviolet light having pluralwavelengths; irradiating said ultraviolet light having pluralwavelengths on a sample on which patterns are formed through anobjective lens while said sample is moving at least in one direction;detecting through said objective lens an image of said patterns on saidsample formed by light reflected from said sample by the irradiation ofsaid ultraviolet light with an image sensor, and adjusting apolarization condition of said light reflected from said sample so as toimprove contrast of said image of said patterns and parallellyoutputting a signal detected by said image sensor wherein the imagesensor is a time delay and integration (TDI) sensor, and wherein aperiod Ts of scanning a pupil is required to be shorter than N times anaccumulation time of one stage of the TDI sensor, where N is an integer;and processing said detected image by using a reference image anddetecting a defect on said sample.
 8. A defect inspection methodaccording to claim 7, wherein said image sensor is a parallel-outputtype image sensor.
 9. A defect inspection method according to claim 7,wherein said image sensor is a rear-side irradiation type in which therear side is sensitive to ultraviolet light.
 10. A defect inspectionmethod according to claim 7, wherein said time delay integration sensoris a rear-side irradiation type in which the rear side is sensitive toultraviolet light.
 11. A detect inspection method according to claim 7,wherein said image sensor is sensitive to a wavelength of less than 400nm.
 12. A defect inspection method, comprising: emitting ultravioletlight having plural wavelengths; obliquely illuminating a sample onwhich patterns are formed with the ultraviolet light having pluralwavelengths through an objective lens while said sample is moving atleast in one direction; detecting through said objective lens an imageof said patterns on said sample formed by light reflected from saidsample by the irradiation with said ultraviolet light, with an imagesensor synchronous with the moving of said sample, while adjusting apolarization condition of the light reflected from said sample so as toimprove contrast of said image of said patterns and parallellyoutputting a signal detected by said image sensor, wherein the imagesensor is a time delay and integration (TDI) sensor, and wherein aperiod Ts of scanning a pupil is required to be shorter than N times anaccumulation time of one stage of the TDI sensor, where N is an integer;and processing the detected image by using a reference image anddetecting a defect on said sample.
 13. A defect inspection methodaccording to claim 12, wherein an oblique illumination on said sample isperformed by scanning said ultraviolet laser beam in a pupil plane ofsaid objective lens.
 14. A defect inspection method according to claim12, wherein said image sensor Is a parallel-output type Image sensor.15. A defect inspection method according to claim 12, wherein said imageis detected by a rear-side irradiation type time delay integrationsensor which Is sensitive to ultraviolet light.
 16. A defect inspectionmethod according to claim 12, wherein said image sensor is sensitive toa wavelength of less than 400 nm.
 17. A defect inspection apparatus,comprising: a light source to emit an ultraviolet light having pluralwavelengths; an optical unit to irradiate a sample on which patterns areformed, with the ultraviolet light having plural wavelengths Through anobjective lens; a table unit to mount said sample arid movable at leastin one direction; an image detecting unit to detect an image of saidpatterns on said sample through said objective lens with an image sensorby extracting a desired polarized component from light reflected fromthe sample by the illumination of the ultraviolet light by adjusting ananalyzer to improve contrast of said image of said patterns andparallelly outputting a signal detected by said image sensor, whereinthe image sensor is a time delay and integration (TDI) sensor, andwherein a period Ts of scanning a pupil is required to be shorter than Ntimes an accumulation time of one stage of the TDI sensor, where N is aninteger; and an image processor to process a detected image by using areference image and to detect a defect on said sample.